Stents, grafts, stent-grafts and similar implantable medical devices, collectively referred to hereinafter as stents, are radially expandable endoprostheses. Stents may be implanted in a variety of body lumens or vessels such as within the vascular system, urinary tracts, bile ducts, etc. Stents are generally tubular devices for insertion into body lumens. Stents are typically delivered via a catheter in an unexpanded configuration to a desired body lumen. Once at the desired location, the stent is deployed and implanted in the body lumen. Typically, a stent will have an unexpanded (closed) diameter for delivery and a deployed (opened) diameter after placement in the body lumen. They may be self-expanding, mechanically expandable or hybrid expandable.
Self-expanding stents are typically implanted in a blood vessel or other body lumen at the site of a stenosis or aneurysm by so-called “minimally invasive techniques” in which the stents are compressed radially inwards and are delivered by a catheter to the site where the stents are required through the patient's skin or by a “cat down” technique in which the blood vessel to be treated is exposed by minor surgical means. Self-expanding stents may be constructed from a variety of materials such as stainless steel, Elgiloy®, nickel, titanium, Nitinol®, Phynox®, shape-memory polymer etc. Stents may also be formed in a variety of manners as well. For example a stent may be formed by etching or cutting the stent pattern from a tube or sheet material; a sheet of metal may be cut or etched according to the desired pattern whereupon the sheet may be rolled or otherwise formed into the desired substantially tubular, bifurcated or other shape of stent; one or more wires or ribbons of stent material may be woven, braided or otherwise formed into a desired shape and pattern. Stents may include components that are welded, bonded or otherwise engaged to one another.
In some systems for the delivering of self-expanding stents, the stent is deployed by a retracting sheath system (i.e., pull-back sheath system). In such technique, the compressed stent is preloaded into a distal portion of a retracting sheath included in the delivery system. The delivery system is driven by an operator from the proximal side on through the vascular system until the distal end of the sheath reaches the implantation site. Then the stent is pushed out from the distal end of the system, and caused or allowed to expand to a predetermined diameter in the vessel. When the stent is constrained within the system, the stent is exerting a force onto the inside diameter (ID) of the sheath. Perceived problems with conventional stent delivery systems include a negative interaction of the sheath with the stent caused by the frictional interface between the stent and sheath which prevents the system from properly deploying the stent.
US 2006/0030923 discloses a stent delivery system comprising a retracting sheath and a roll-back inner membrane having a lubricious coating. The inner membrane is disposed directly around a stent and the sheath is disposed around the membrane. The distal end of the membrane is engaged to a distal portion of the sheath and proximal end of the membrane is engaged to a portion of an inner catheter shaft proximal of a stent retaining region of the delivery system. Since the inner membrane rolls back proximally along the length of the stent until the stent is fully exposed and deployed when the pull-back sheathe is retracted, the frictional interface between the stent and sheath is reduced and thus the stent can be properly deployed. However, since the membrane must cover at least all length of the stent within the delivery system, a total volume of the components included in the system becomes bulky and a diameter of the delivery system has to be greater than a diameter of the system without the membrane. It will reduce flexibility and usability of the delivery system.
Perceived problems also include “stent-jumping” which is longitudinal displacement of a self-expending stent, when a retracting sheath is withdrawn from the stent. It occurs because expansion force of the stent is greater than the stent frictional force and stent constraint force at an angle exiting the system.
US 2004/0204749 discloses a stent delivery system comprising a shaft having a plurality of protrusions extending radially outward from a surface thereof. A proximal portion of a stent loaded into the system is temporarily engaged to the protrusions until the engaged portion of the stent is freed to expand. This engagement prevents the stent from moving longitudinally relative to the system (i.e., stent jumping) by controlling the expansion force of the stent at an angle exiting the system during placement of the stent. In order to increase delivery accuracy, a stent delivery system is desired not only to prevent “stent jumping” but also to comprise a re-sheathing function which allows a partially unsheathed stent to be drawn back into the delivery system for repositioning. The protrusions discloses in US 2004/0204749 is not able to re-sheath the partially unsheathed stent because the protrusions do not provide sufficient holding force to make an efficient re-sheathing movement.
EP0775470 A1 discloses a stent delivery device having a scratch protection means for preventing a vessel from being dangerously scratched and perforated by the edges of stent. In a delivery configuration, the scratch protection means is positioned in the sheath and partially engaged to the proximal end of the stent. Since the scratch protection means is formed by a tube of thermoformable material heat shrunk on the shaft but not of self-expanding property, sufficient holding force to make an efficient re-sheathing movement for accurate deployment cannot be expected.
WO 2011/014814 discloses a stent delivery system comprising a pair of forceps-like holders and a middle bumper disposed on an inner shaft. A stent is pinched between the holders and the middle bumper within an outer sheath of the system by keeping the proximal ends of the holders in a hypotube, which is disposed about the shaft, during placement. At the desired place, the stent can be unsheathed and, if necessary, it can be re-sheathed by sliding back the sheath over the stent until the stent is released from the holders and the middle bumper by retracting the hypotube proximally and putting holders in their open position. Although this system may improve the delivery accuracy by providing the re-sheathing function, via the pair of forceps-like holders and the middle bumper, it has poor flexibility because of the rigid property of the holder and middle bumper required for ensuring a sufficient holding ability. Furthermore, because of the additional component like hypotube for making the pinching action of the holders, the system becomes more bulky (the diameter of the system becomes greater) and less flexible. It fatally reduces the usability of the stent delivery system, especially for tiny vessels.
US2011/0082464 discloses an implant delivery system having a first expandable means for loading a polymeric tubular implant into the system without damage. This first expandable means is attached to the distal end of an inner shaft. The distal end of this first expandable means is designed to be engaged to and surrounding a proximal end of the implant upon loading this implant into the delivery system. Since the first expandable means is for loading, once the implant is loaded into the system, the first expandable means is pulled away and does no longer cover any part of the implant. Therefore, US2011/0082464 discloses neither a delivery configuration including the first expandable means nor any effective overlap ratio of the implant with the first expandable means within an outer sheath for improving deployment accuracy. US2011/0082464 further discloses an enlarged diameter portion positioned on the inner shaft and mentions that its use may be helpful in withdrawing the first expandable member over the implant because it can prevent the implant from being dragged proximally when the first expandable means is withdrawn from the delivery system after the loading of the implant. US2011/0082464, however, fails to disclose a delivery configuration comprising an enlarged diameter portion positioned within the first expandable means.
US2007/0270932 also discloses a system for loading a stent into a delivery system with an engaging member having an open distal end as stent holding means. Again, since this engaging member is designed for loading, US2007/0270932 fails to disclose a solution for obtaining a lower profile of a delivery system while keeping adequate deployment accuracy.
U.S. Pat. No. 8,048,139 discloses a stent delivery system with an expendable braided bumper as stent holding means. The braided bumper is joined to a shaft (i.e., a pusher) disposed within a retracting sheath and a self-expanding stent is disposed around the bumper and the shaft. By using the expansion force exerted by the bumper onto the inner surface of the stent within the retracting sheath, the partially deployed stent can be drawn back. The braided structure occupies only a small diameter when folded up and thus provides flexibility during placement. However, since the system utilizes the expansion force of the bumper in order to hold a proximal portion of the stent, it increases the undesired frictional force between an inner surface of the sheath and an outer surface of stent, resulting in increasing the risk of undesired proximal shifting of stent during the deployment step.